Hydrodynamic Study of a Column Flotation usingElectrical Resistance Tomography (ERT) and CFDTechniques
2017
Column flotation has been widely used in mineral processing and coal preparation
industry. Although the basic concept of column flotation is relatively simple, the
fundamental principles governing column performance are complex and not easily
quantifiable. Performance of a column flotation mainly depends on the hydrodynamic
parameters of the process such as gas dispersion, which includes bubble size
distribution (BSD), gas hold-up, air superficial velocity (Jg) and bubble surface-area
flux (Sb). Because of its high speed competence, low price measurement, robust
sensors and non-intrusive nature, Electrical resistance tomography (ERT) is
considered to be the most powerful tool compare with other techniques. Apart from
experimental methods, there are computational models based on fluid mechanics, if
developed for a specific process, would offer better understanding of multi-phase
interactions especially for column flotation. The complete understanding of
hydrodynamics of column flotation is essential to model and develop design criteria.
However its similarity to the bubble column provides a basis for improved
understanding of fluid flow in the column flotation.
In this work we intend to develop a CFD model for hydrodynamics of column
flotation process mainly two-phase (air-water) flow followed by extending to three
phase flow (air-water-solids) system. To test the reliability of the CFD models, this
work also focuses on experimental investigation of column hydrodynamics using high
speed ERT, pressure transducers (PT) and high speed video camera (HSVC).
Tomography experiments are carried out to analyse the gas-liquid two phase
flow behaviour in fabricated bubble column and column flotation. The gas dispersion
characteristics in a 100 mm laboratory column flotation have been investigated in
terms of the local and mean gas hold-up, bubble rise velocity and bubble size
distribution across the column. This experimental data has been used to demonstrate
the validation of the two-fluid CFD model predictions in the column flotation. Using
the ERT system, measurement of two phase distributions are examined for a wide
range of design and operating conditions of the column including different spargers
and frother dosage, where the flow changes from homogenous to transition bubbly
flow. It is confirmed by ERT that the gas-holdup increases with an increase in the
viii
sparger porosity, air superficial velocity, liquid height and liquid feed flow rate.
Dynamic gas disengagement technique (DGD) coupled with ERT is utilized to
measure bubble rise velocity and sauter mean bubble diameter.
For three phase flow of column flotation, the distribution characteristics of
combined solid and gas hold-up is studied using ERT coupled with pressure
transducers (PT) in the column flotation. The effect of superficial gas velocity, feed
slurry flow rate, slurry height, sparger pore number density and frother dosage in the
column on mean gas hold-up and its radial distribution has been analysed for three
phase systems. Mean gas and solids hold-ups extracted from ERT have been critically
assessed for the column operating in various flow regimes. The results show that gas
hold-up (eg) increases in the column with an increase in the Jg, whereas solids holdup distribution is very homogeneous for high gas velocities. The presence of solids
renders the bubble rise velocity thereby decreases the local gas hold-up. Further feed
slurry flow rate and frother effects on column hydrodynamics have been explored and
quantified. Some contradictions are observed in the literature regarding gas hold-up
change in the presence of solids. Using the DGD method, the effect of solids on gas
hold-up has been assessed in terms of bubble rise velocity and sauter mean bubble
diameter. DGD would assume that the solids radial distribution is invariant in axial
direction in the column. The bubble sauter mean diameter is increased in the presence
of solids, indicating possibility of the bubble coalescence. Due to coalescence
phenomena, the gas hold-up is less in presence of solids compare with without solids.
Initially CFD studies were carried out in rectangular bubble column and in house 100
mm fabricated bubble column. Two-fluid model (TFM) with k-e turbulence model is
employed in these simulations. Suitable interphase forces accounting for virtual mass,
lift and drag force are tested and evaluated for bubble plume evaluation and gas
dispersion dynamics. Different drag models based on bubble shape and size are tested
for correct flow dynamics. Population balance method is coupled with two fluid model
to evaluate the evolving transitional flow behaviour by consider bubble break-up and
coalescence sub-processes. The liquid-phase velocities and gas hold-up predictions
are validated against the measured data in a bubble column. The simulation values are
matches well matched with Buwa and Ranade (2003) experimental data for different
air superficial velocities.
Numerical simulations have been carried out to analyse the hydrodynamic parameters
such as mean gas hold-up, axial liquid velocity, sauter mean diameter, and number
density fraction for column flotation. The simulations are performed for two CF; 100
mm fabricated laboratory column flotation and literature based 500 mm column
flotation. PBM has been used to consider the bubble interactions at various operational
parameters such as air superficial velocity, liquid height, sparger pore density and
liquid velocity. Simulations also being carried out without PBM. Implementation of
PBM method leads to better agreement with experimental data especially in the
evolving transitional flow regime (1.2 & 1.8 cm/s air superficial velocity). Because of
homogeneous bubbly flow at low air superficial velocities (up to 0.9 cm/s), the
predicted mean gas hold-up values are well matched with and without PBM
simulation values. The predicted hydrodynamic parameters are validated with ERT
experimental values.
Three phase CFD simulations are also performed in column by considering
hydrophilic silica as a solid phase similar to experiments. Euler-Euler model with
three phases coupling with PBM model has been used to explore the hydrodynamics.
The solid and gas radial distribution and effect of solids concentration on combined
hold-up are investigated in the column. The effect of operational parameters such as
air superficial velocity on gas and solid hold-up have been estimated. The predicted
numerical simulations values are validated with ERT experimental data. The
simulations values are well matched with ERT coupling with PT experimental data.
Keywords:
- Correction
- Cite
- Save
- Machine Reading By IdeaReader
0
References
0
Citations
NaN
KQI